This research program was initiated by the observations that the concentrations of plasma amino acids in human and rat sera normally undergo characteristic daily fluctuations as a result of food consumption, and that these changes in plasma amino acid pattern cause corresponding changes in brain tryptophan levels and in the rate at which brain neurons synthesize the neurotransmitter serotonin from tryptophan. Several years ago, with support from AM-14228, we also found that food-induced changes in plasma amino acid pattern can, by varying brain tryosine levels, influence the synthesis and release of the catecholamine neurotransmitters dopamine and norepinephrine. Studies performed during the past year have focussed on increasing our knowledge of the ways that amino acid availability affects brain composition and behavior in rats, and on carrying out the preliminary studies necessary before tyrosine and other amino acids can be tested in the treatment of neuronal diseases related to excessive or inadequate catecholamine release. Specifically, we have: 1) Characterized the range of normal plasma neutral amino acid levels, as functions of time of day and of protein consumption, in humans; 2) Developed a model which predicts the extent to which tryptophan's binding to albumin might or might not affect its availability to the brain (The effect of albumin binding almost always is small...); 3) Examined the relation between dietary tryptophan and serotonin synthesis in the gut; 4) Initiated studies on the control by central serotoninergic neurons of peripheral sympathetic functions; 5) Extended our understanding of the particular effects of brain serotonin on appetite; and, 6) Extended our knowledge of tyrosine's effects on rat brain and behavior, and started studies on its effects on normal human subjects.